Fletcher, who had started Scotland's last two fixtures, was carried off in the Owls' draw with Derby County on Saturday after suffering ankle/knee injuries.
But he shrugged off the knocks in time to face Bolton Wanderers on Tuesday evening. The 31-year-old bagged the opener in the Owls' hard-fought victory at the University of Bolton Stadium.
Fletcher, who underwent major knee surgery last term, is in a rich vein of form, having notched four goals in his last five Wednesday outings.
"He's got ongoing issues that him and the club feel need to be addressed," said McFadden.
"He's managing an injury and the plastic pitch in Kazakhstan wouldn't have helped. He's not long back from being out for eight months after operations on his ankle and his knee."
Following a 13-month absence from international duty, Fletcher scored a penalty on his return in November's 4-0 success over Albania. He then claimed his 32nd cap against Israel a few days later.
Liam Palmer and Barry Bannan, Fletcher's Wednesday team-mates, have been selected in the Scotland squad by boss Alex McLeish.
The Scots travel to Kazakhstan and San Marino later this month.